1. Field of the Invention
The present invention relates to a pressure-pulse-wave detecting apparatus which presses an artery of a living subject and detects a pressure pulse wave produced from the artery.
2. Related Art Statement
A pressure-pulse-wave detecting apparatus which includes a pressure-pulse-wave sensor, presses the sensor against an artery of a living subject via skin, and detects a pressure pulse wave produced from the artery, is employed in, e.g., a blood-pressure monitoring apparatus which successively determines blood-pressure values of the subject based on the pressure pulse wave, or a pulse-wave-propagation-velocity measuring apparatus which measures a pulse-wave propagation velocity based on at least the pressure pulse wave.
In the above-mentioned pressure-pulse-wave detecting apparatus, a magnitude of the pressure pulse wave detected by the pressure-pulse-wave sensor largely depends on a pressing force applied to the sensor, and additionally a phase of the pressure pulse wave more or less depends on the pressing force. Therefore, the pressure-pulse-wave detecting apparatus needs to press the artery with a pressing force (hereinafter, this pressing force will be referred to as an xe2x80x9coptimumxe2x80x9d pressing force) which assures that a portion of the wall of the artery is flattened. Meanwhile, usually, the pressure-pulse-wave sensor has a plurality of pressure-detecting elements that are arranged in an array in a widthwise direction of the artery, so that even if the sensor is more or less moved relative to the artery, at least one of the pressure-detecting elements remains positioned right above the artery to be able to detect the pressure pulse wave.
A method of determining the above-mentioned optimum pressing force is disclosed in, e.g., Japanese Patent Document No. 11-9562. This document discloses a blood-pressure monitoring apparatus including the above-explained pressure-pulse-wave detecting apparatus, and teaches that the pressure-pulse-wave sensor is worn on a wrist to detect a pressure pulse wave produced by a radial artery. According to this document, the optimum pressing force applied to the pressure-pulse-wave sensor is determined as follows: First, the pressing force applied to the sensor is continuously increased up to a value at which the entirety of the wall of the radial artery is flattened. Then, one of the pressure-detecting elements that detects, during the continuous increasing of the pressing force, the highest pressure of the respective pressures detected by all the elements is selected (as a highest-pressure detecting element), and a pressing force at which the pressure pulse wave detected by the selected element exhibits a maximal amplitude, or a pressing force that falls within a range whose center is equal to that pressing force, is determined as the optimum pressing force.
In the case where a pressure-pulse-wave sensor of, e.g., the pressure-pulse-wave detecting apparatus disclosed in the above-mentioned document, is worn on a wrist to detect a pressure pulse wave from a radial artery, the radial artery is sandwiched between the sensor and a radius. Therefore, as the pressing force applied to the sensor is increased, the radial artery can be substantially completely flattened. That is, since the radial artery is supported on the radius, the artery can be completely flattened. However, there are many arteries that are not supported on such bones. That is, if the pressing force applied to the sensor pressing an artery not supported on a bone is continuously increased, the artery cannot be completely flattened. Thus, in that case, an appropriate pressing force which causes only a portion of the wall of the artery to be flattened may not be determined.
It is therefore an object of the present invention to provide a pressure-pulse-wave detecting apparatus which can determine an appropriate pressing force that causes only a portion of a wall of an artery to be flattened, without needing to flatten an entirety of the wall of the artery.
The Inventor has carried out extensive studies to achieve the above object, and has found that in a state in which the pressing force applied to the pressure-pulse-wave sensor causes a portion of a wall of an artery to be flattened, there is no phase difference between respective pressure pulse waves detected by respective pressure-detecting elements of the sensor that are positioned right above the flattened portion of the arterial wall, but there is a phase delay of a pressure pulse wave detected by a pressure-detecting element positioned right above a non-flattened portion of the arterial wall, from the phase of the pressure pulse waves detected by pressure-detecting elements positioned right above the flattened portion, owing to a visco-elasticity of the non-flattened portion. Thus, if a phase of a pressure pulse wave detected by a pressure-detecting element distant by a prescribed distance from the highest-pressure detecting element in a direction toward an end of the array of pressure-detecting elements, e.g., an pressure-detecting element next to the highest-pressure detecting element, is not delayed from the phase of the pressure pulse wave detected by the highest-pressure detecting element, or if the former phase is later than the latter phase by a short time only, it can be judged that a portion of the wall of the artery is flattened.
In addition, the Inventor has found that there is substantially no pulse-pressure difference between the respective pressure pulse waves detected by the respective pressure-detecting elements positioned right above the flattened portion of the arterial wall, but a pulse pressure of the pressure pulse wave detected by the pressure-detecting element positioned right above the non-flattened portion of the arterial wall is smaller than the pulse pressures of the pressure pulse waves detected by pressure-detecting elements positioned right above the flattened portion, owing to a pressure loss caused by the visco-elasticity of the non-flattened portion. Thus, if a pulse pressure of the pressure pulse wave detected by the pressure-detecting element distant by the prescribed distance from the highest-pressure detecting element in the direction toward the end of the array of pressure-detecting elements, is substantially equal to the pulse pressure of the pressure pulse wave detected by the highest-pressure detecting element, it can be judged that a portion of the wall of the artery is flattened. The present invention has been developed based on these findings.
The above object has been achieved by the present invention. According to a first aspect of the present invention, there is provided an apparatus for detecting a pressure pulse wave produced by an artery of a living subject, comprising a pressure-pulse-wave sensor which has a pressing surface, and a plurality of pressure-detecting elements that are arranged, in the pressing surface, in an array in a widthwise direction of the artery; a pressing device which presses, with a pressing force, the pressure-pulse-wave sensor against the artery via a skin of the subject, so that each of the pressure-detecting elements detects the pressure pulse wave produced by the artery; a highest-pressure-detecting-element selecting means for selecting, as a highest-pressure-detecting element, a first one of the pressure-detecting elements that detects a highest one of respective pressures corresponding to the respective pressure pulse waves detected by the pressure-detecting elements; and a pressing-force checking means for judging whether the pressing force of the pressing device applied to the pressure-pulse-wave sensor is appropriate, based on a time difference between a first time when a prescribed portion of the pressure pulse wave is detected by the highest-pressure-detecting element and a second time when the prescribed portion of the pressure pulse wave is detected by a second one of the pressure-detecting elements that is distant by a prescribed distance from the highest-pressure-detecting element in a direction toward one of opposite ends of the array of pressure-detecting elements.
According to this aspect, the pressing-force checking means judges whether the pressing force applied to the pressure-pulse-wave sensor is appropriate, based on the time difference between the time when the prescribed portion of the pressure pulse wave is detected by the highest-pressure-detecting element and the time when the prescribed portion of the pressure pulse wave is detected by the pressure-detecting element distant by the prescribed distance from the highest-pressure-detecting element in the direction toward one end of the array of pressure-detecting elements. Thus, the present apparatus can determine an appropriate pressing force that causes a portion of the wall of the artery to be flattened, without needing to substantially completely flatten the arterial wall.
According to a second aspect of the present invention, there is provided an apparatus for detecting a pressure pulse wave produced by an artery of a first portion of a living subject, comprising a pressure-pulse-wave sensor which has a pressing surface, and a plurality of pressure-detecting elements that are arranged, in the pressing surface, in an array in a widthwise direction of the artery; a pressing device which presses, with a pressing force, the pressure-pulse-wave sensor against the artery via a skin of the subject, so that each of the pressure-detecting elements detects the pressure pulse wave produced by the artery of the first portion; a heartbeat-synchronous-signal detecting device which detects a heartbeat-synchronous signal produced from a second portion of the subject that is different from the first portion; a highest-pressure-detecting-element selecting means for selecting, as a highest-pressure-detecting element, a first one of the pressure-detecting elements that detects a highest one of respective pressures corresponding to the respective pressure pulse waves detected by the pressure-detecting elements; a standard-pulse-wave-propagation-velocity-related-information obtaining means for obtaining, as a piece of standard pulse-wave-propagation-velocity-related information, a first piece of pulse-wave-propagation-velocity-related information that is related to a velocity at which the pressure pulse wave propagates between the first and second portions of the subject, based on a first time difference between a first time when a prescribed portion of the pressure pulse wave is detected by the highest-pressure-detecting element and a second time when a corresponding prescribed portion of the heartbeat-synchronous signal is detected by the heartbeat-synchronous-signal detecting device; a comparison-pulse-wave-propagation-velocity-related-information obtaining means for obtaining, as a piece of comparison pulse-wave-propagation-velocity-related information, a second piece of pulse-wave-propagation-velocity-related information that is related to the velocity at which the pressure pulse wave propagates between the first and second portions of the subject, based on a second time difference between a third time when the prescribed portion of the pressure pulse wave is detected by a second one of the pressure-detecting elements that is distant by a prescribed distance from the highest-pressure-detecting element in a direction toward one of opposite ends of the array of pressure-detecting elements, and the second time when the corresponding prescribed portion of the heartbeat-synchronous signal is detected by the heartbeat-synchronous-signal detecting device; and a pressing-force checking means for judging whether the pressing force of the pressing device applied to the pressure-pulse-wave sensor is appropriate, based on a comparison value obtained by comparing the piece of standard pulse-wave-propagation-velocity-related information and the piece of comparison pulse-wave-propagation-velocity-related information with each other.
According to this aspect, the standard-pulse-wave-propagation-velocity-related-information obtaining means obtains the piece of standard pulse-wave-propagation-velocity-related information based on the pressure pulse wave detected by the highest-pressure-detecting element and the heartbeat-synchronous signal detected by the heartbeat-synchronous signal detecting device, and the comparison-pulse-wave-propagation-velocity-related-information obtaining means obtains the piece of comparison pulse-wave-propagation-velocity-related information based on the pressure pulse wave detected by the pressure-detecting element distant by the prescribed distance from the highest-pressure-detecting element in the direction toward one end of the array of pressure-detecting elements and the heartbeat-synchronous signal detected by the heartbeat-synchronous-signal detecting device. Since both the piece of standard pulse-wave-propagation-velocity-related information and the piece of comparison pulse-wave-propagation-velocity-related information are obtained based on the prescribed portion of the heartbeat-synchronous signal, as one of two reference points, a difference between the two pieces of information is caused by a time difference between the time when the prescribed portion of the pressure pulse wave is detected by the highest-pressure-detecting element and the time when the prescribed portion of the pressure pulse wave is detected by the pressure-detecting element distant by the prescribed distance from the highest-pressure-detecting element. Therefore, the pressing-force checking means can judge whether the pressing force applied to the pressure-pulse-wave sensor is appropriate, based on the comparison value obtained by comparing the two pieces of information with each other. Thus, the present apparatus can determine an appropriate pressing force that causes a portion of the wall of the artery to be flattened, without needing to substantially completely flatten the arterial wall.
According to a third aspect of the present invention, there is provided an apparatus for detecting a pressure pulse wave produced by an artery of a living subject, comprising a pressure-pulse-wave sensor which has a pressing surface, and a plurality of pressure-detecting elements that are arranged, in the pressing surface, in an array in a widthwise direction of the artery; a pressing device which presses, with a pressing force, the pressure-pulse-wave sensor against the artery via a skin of the subject, so that each of the pressure-detecting elements detects the pressure pulse wave produced by the artery; a highest-pressure-detecting-element selecting means for selecting, as a highest-pressure-detecting element, a first one of the pressure-detecting elements that detects a highest one of respective pressures corresponding to the respective pressure pulse waves detected by the pressure-detecting elements; a standard-pulse-pressure determining means for determining, as a standard pulse pressure, a pulse pressure of the pressure pulse wave detected by the highest-pressure-detecting element; a comparison-pulse-pressure determining means for determining, as a comparison pulse pressure, a pulse pressure of the pressure pulse wave detected by a second one of the pressure-detecting elements that is distant by a prescribed distance from the highest-pressure-detecting element in a direction toward one of opposite ends of the array of pressure-detecting elements; and a pressing-force checking means for judging whether the pressing force of the pressing device applied to the pressure-pulse-wave sensor is appropriate, based on a comparison value obtained by comparing the standard pulse pressure and the comparison pulse pressure with each other.
According to this aspect, the standard-pulse-pressure determining means determines, as the standard pulse pressure, the pulse pressure of the pressure pulse wave detected by the highest-pressure-detecting element, and the comparison-pulse-pressure determining means determines, as the comparison pulse pressure, the pulse pressure of the pressure pulse wave detected by the pressure-detecting element distant by the prescribed distance from the highest-pressure-detecting element in the direction toward one end of the array of pressure-detecting elements. Since a fact that the standard pulse pressure and the comparison pulse pressure are substantially equal to each other indicates that a portion of the wall of the artery is flattened, the pressing-force checking means can judge whether the pressing force applied to the pressure-pulse-wave sensor is appropriate, based on the comparison value obtained by comparing the standard pulse pressure and the comparison pulse pressure with each other. Thus, the present apparatus can determine an appropriate pressing force that causes a portion of the wall of the artery to be flattened, without needing to substantially completely flatten the arterial wall.